Page 35 - HVAC Pump Handbook

Rishel_CH02.qxd 20/4/06 5:13 PM Page 32

Physical Data for HVAC System Design

32 The Basic Tools

The most popular power for HVAC applications is 480 V, three-
1
phase. Single-phase power is seldom used above 7 ⁄ 2 hp. The 208 V
service is derived from a Y-connected transformer in the building
being served; three-phase motors as high as 60 hp are available for
this voltage. The higher voltages of 2400 and 4160 V are used generally
on motors of 750 hp and larger.
Electrical machinery such as motors and variable-speed drives have
specified voltage tolerances that exceed those of the electrical utility.
The electrical design engineer must develop the building power distri-
bution to ensure that its voltage drop does not exceed the voltage tol-
erances of the electrical equipment. Typically, the voltage tolerance of
most electric motors is 10%, and those for most variable-speed dri-
ves appear to be 10 percent and 5 percent. The actual tolerances
for this equipment should be verified by the HVAC water system de-
signer. For example, the utility voltage at a building transformer may
be 480 V 5 percent, or 456 to 504 V. A 460-V variable-speed drive
has an allowable voltage variation of 437 to 506 V. Therefore, the
building power distribution system must be designed so that the
power supply to the variable-speed drive does not drop below 437 V
under any load condition.
Power factor correction equipment can be required by public utilities
or state law above a certain size of motor. This should be checked by the
designer at the beginning of the development of a specific project.
Generally, public utilities do not require power factor correction at most
places in their electrical distribution until the load approaches 500 kVA.
The popularity of the variable-frequency drive has created a prob-
lem for public utilities. This is the harmonic distortion caused by the
alteration of the sine wave by the variable-frequency drive. The public
utility furnishing power on a project may have a specification on the
maximum allowable harmonic distortion. Also, the owner of the facility
may have tolerances on harmonic distortion.
More information on power factor correction and harmonic distor-
tion is included in Chap. 7.

2.9 Efficiency Evaluations of HVAC
Water Systems
Several expressions of efficiency will be provided in the following
chapters that relate to the effectiveness of pump selection and appli-
cation. These will include:

1. System efficiency, which determines the quality of use of pump
head in a water system. This will be expressed as a percentage,
coefficient of performance, kW/ton, or kW/1000 mbh.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.